Simple and compact laser wavelength locker

ABSTRACT

A wavelength locker for monitoring the wavelength drift of a laser uses a pair of detectors for detecting a power component of the laser beam and a wavelength component of the laser beam, respectively. Various positionings of the power detector and/or variations to the collimating lens provide a compact arrangement with fewer components.

FIELD OF THE INVENTION

Embodiments of the present invention are directed to wavelength lockersand, more particularly, embodiments of the present invention aredirected to more compact wavelength lockers conserving valuable packagespace.

BACKGROUND INFORMATION

Wavelength division multiplexing (WDM) is a technique used to transmitmultiple channels of data simultaneously over the same optic fiber. At atransmitter end, different data channels are modulated using lighthaving different wavelengths or, colors for each channel. The fiber cansimultaneously carry multiple channels in this manner. At a receivingend, these channels are easily separated prior to demodulation usingappropriate wavelength filtering techniques.

The need to transmit greater amounts of data over a fiber has led toso-called Dense Wavelength Division Multiplexing (DWDM). DWDM involvespacking additional channels into a given bandwidth space. The resultantnarrower spacing between adjacent channels carried by a fiber in DWDMsystems demands precision wavelength accuracy from the transmittinglaser diodes.

Unfortunately, as laser diodes age, they are known to exhibit awavelength drift of up to 0.15 nm from their set frequency over about afifteen year period. This period is well within the expected servicelife of modern laser diodes. Hence, this wavelength drift isunacceptable as a given channel may drift and interfere with adjacentchannels causing cross talk. To remedy this situation most lasertransmitters use what is commonly referred to in the art as a wavelengthlocker to measure drift frequency vs. set frequency. This informationcan be fed back to a controller to adjust various parameters, such astemperature or drive current, of the laser diode to compensate for theeffects of aging and keep the diode laser operating at its setfrequency. Most laser transmitters with an integrated wavelength lockeruse either an etalon or thin film filter to measure the laser wavelengthvariation.

FIGS. 1A and 1B show a type of conventional wavelength lockerconfiguration. A laser 6 produces a laser beam centered about a setfrequency or wavelength. The laser 6 emits a light beam from both afront facet 15 and a back facet 13. The actual modulated light carryingthe data channel emerges from the front facet 16, which is coupled to anoptical fiber (not shown). The beam 12 that emerges from the back facet13 is used for monitoring purposes since it has the same wavelength asthe beam emerging from the front facet 15. The monitored beam 12 passesthrough a lens 8. A beam splitter 10 splits a monitored beam 12 into twobeams. The first beam 14 passes through the splitter 10 and is receivedby a first detector 16, hereinafter referred to as the power monitordetector 16. The second beam 20 is deflected and passes through awavelength filter (etalon) 22 after which it is received by a seconddetector 24, hereinafter referred to as the filter detector 24.

In operation, the detectors 16 and 24, which may be for example,photodiode or optoelectrical detectors, output an electric signal basedon the optical input of the received beam. The first detector 16receives the first beam 14 and outputs a signal that is a function ofthe monitored beam's 12 power. The second detector 24 receives thesecond beam 20 and outputs a signal that is a function of both themonitored beam's 12 power as well as its wavelength. Thus, bymathematically operating on these signals as output by the detectors,16and 24, the wavelength of the monitored laser beam 12 can be determinedand compared to the set frequency to determine any wavelength drift ofthe laser's 6 output.

The above configuration includes a beam splitter 10 as well as a filter22 and second detector 24, positioned perpendicular to the optical axisof the monitored beam 12. Thus, this arrangement takes up an undesirablylarge amount of space in an optical device package.

FIG. 2 shows an alternate wavelength locker configuration that uses a“stacked” arrangement of detectors. As shown, the filter detector 26 andthe power monitor detector 27 are stacked one on top of the other with afilter 28 placed in front of the filter detector 26. A collimated beam29 strikes both of the detectors, 26 and 27 with the lower portion ofthe beam 29 first passing through the filter 28 prior to striking thefilter detector 26. Unfortunately, in this configuration the centerportion of the collimated beam 29 where the power of the beam is thehighest is not used. Thus, this configuration is not as sensitive todetect small changes in the beam as is desired.

Since optoelectronics packaging is one of the most difficult and costlyoperations in the manufacturing process, designers are always strivingfor simpler more compact cost effective arrangements and solutions.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, wherein likenumerals indicate like elements throughout:

FIG. 1A is a plan view of conventional configuration for a wavelengthlocker;

FIG. 1B is a block diagram of the wavelength locker shown in FIG. 1A;

FIG. 2 is a block diagram of a conventional stacked detector wavelengthlocker;

FIG. 3 is a block diagram of a wavelength locker according to oneembodiment of the invention;

FIG. 4 is a plan view of wavelength locker according to one embodimentof the invention;

FIG. 5 is a diagram plotting the filter response for various placementsof the power monitor detector;

FIG. 6 is a block diagram of a wavelength locker according to anotherembodiment of the invention;

FIGS. 7A-7B show plan views of yet another embodiment of the inventionusing the GRIN lens as both a collimator and a beam splitter; and

FIG. 8 is a ray tracing diagram showing the operation of the GRIN lensof FIGS. 7A-7B.

DETAILED DESCRIPTION

One embodiment of the present invention is shown in FIG. 3. Here, a backfacet 30 of a laser diode 32 outputs a monitored beam 34. The monitoredbeam 34 passes through a lens 36 to produce a collimated beam 38. Thecollimated beam 38 passes through a filter (etalon) 40 and thereafterthe now collimated, filtered beam 42 falls on a filter detector 46 whichoutputs a signal indicating the power of the beam 34 as well as thewavelength of the beam being output by the laser diode 32.

Unlike the conventional examples shown in FIGS. 1A and 1B, no beamsplitter is used. Instead, the second detector 48 is placed directly inthe path of the monitored beam in front of the lens 36. The signalsoutput by the detectors, 46 and 48, can be mathematically operated on todetermine the wavelength of the monitored beam 34.Two cases for possibleplacement of the second detector are shown in FIG. 3. In the first case(case 1), the power monitor detector 48 is centered in the path of themonitored beam 34 about 10 μm behind the laser 32. In the second case(case 2) the power monitor detector 48′ is placed about 30 μm behind thelaser and offset to one side by about 10 μm.

FIG. 4 shows a set up for testing the impact of a detector between thelens 36 and the laser 32 on the etalon 40 to measure the etalonresponse. As shown, the set up comprises a laser diode 32 mounted on asubstrate 31. The power monitor detector 48 is also mounted on thesubstrate behind the laser diode 32. A collimating lens 32 collimatesthe light from the laser 32 which is then filtered by filter 40 and isdetected by the filter detector 46.

Measurements were taken with the power monitor photodiode 48 placed attwo different locations as discussed above. For the first measurement,the power monitor photodiode 40 was placed approximately 10 μm behindthe laser diode 32. For the second measurement, the power monitorphotodiode 40 was placed approximately 30 μm behind and 10 μm to theside of the laser diode 32. In both cases, sufficient light wascollected by filter detector 46 for the wavelength locker to operatewithin acceptable specifications. For the disclosed embodiments aminimum signal strength of 20 μA output by the filter detector 36 isrequired for effective wavelength locking. In the first case, the lightcollected produced a 136 μA signal output from the filter detector 46.In the second case, a 72 μA signal was produced from the collected lightby the filter detector 46. Both, well within the acceptable range.

In addition to signal strength, the extinction ratio (ER) is also afactor that needs to be considered. When positioning the power monitordetector 40 in the direct path of the monitored laser beam it blockssome of the light that would otherwise pass through the etalon 40 andreach the filter detector 46. The extinction ratio (ER) is a measure ofthe effectiveness of the etalon filter for wavelength locking. Theextinction ratio is defined as:

ER=(Maximum filter detector current)/(minimum filter detector current).The minimum ER specification for the disclosed embodiments is 3 dB.

As shown in FIG. 5, without the power monitor detector 48 partiallyblocking the path of the laser, the measured ER was 4.9 dB. With thedetector 10 μm behind the laser, the measured ER was 4.3 dB. Finally,with the detector 30 μm behind the laser 32 and 10 μm to the side of thelaser 32 a higher ER of 5.3 dB was measured. These measurements areshown in FIG. 5 which again demonstrates that a sufficient ERmeasurement can be obtained. In particular, it is noted that there is noappreciable change in etalon response as the power monitor detector 48is repositioned between the etalon 40 and the laser 32.

This embodiment of the invention eliminates the need for a beam splitteras well as reduces the overall footprint of the wavelength locker savingpackage space. Of course, the examples offered show the power monitordetector 48 in two alternate positions; however, it is understood bythose skilled in the art that the power detector 48 could be anywherewithin the area of the beam 34 so long as sufficient light can begathered by the detectors 40 and 46. For example, the power detector maybe positioned 5-15 μm behind the laser 32 and 20-40 μm to the side ofthe laser 32.

FIG. 6 shows another embodiment of the invention that uses a lens havingan angled, polished face to split the monitored beam between the twodetectors. As shown, the back facet 60 of a laser diode 62 outputs amonitored beam 64 which is collimated through a micro-gradient index(GRIN) lens 65. The end face 66 of the GRIN 65 is angled at 45 degreesand is coated with a broadband partially reflective coating. Of courseother angles may be appropriate such as in a range between 30-60degrees. The GRIN lens 65 used in this fashion permits the use of asingle element as both a collimator and a splitter.

The splitting ratio can be selected by the appropriate selection of thecoating material. For example, a coating may be selected to provide for30% transmission and 70% reflection of passing light. A thin film filter67 filters the reflected beam. The power monitor detector 68 gives asignal (signal 1) proportional to power only and the filter detector 69gives a signal (signal 2) that is a function of wavelength and power. Asbefore, by mathematically operating on these two signals, as withcontroller 61, the wavelength of the monitored beam 64 can bedetermined.

Alternatively, the filter 67 can be omitted and instead, a thin filmfilter 65 can be applied directly on the GRIN end face 66. In this case,both detectors, 67 and 68, produce a signal having a function ofwavelength since filtered light reaches both detectors. In this case,the sum of the two signals can be used to monitor the power of thelaser. Further, in this alternate arrangement, the difference of the twodetector signals has twice the slope vs. wavelength compare the casewhen the filter 67 is used, effectively enhancing the wavelength lockersensitivity.

FIGS. 7A-B show yet another embodiment of the present invention similarto the embodiment shown in FIG. 4. A laser 70 is mounted on a sub-mount71 on a substrate 72. A monitored beam from the back facet of the laser70 is collimated with a GRIN lens 73. A thin film reflective coatingfilter 74 is placed on the far end of the GRIN lens 73 that allows aportion of the monitored beam to pass through. As shown in FIG. 7A theportion of the monitored beam that passes through is filtered by afilter 75 and then passes to the filter detector 76. In FIG. 7B, thefilter 75 is replaced by a thin film filter 75′ also coating the GRINlens 73 However, unlike the previous embodiments, the power detector 77is placed adjacent to the laser 70 since a second portion of themonitored light is reflected back through the GRIN lens by the thin filmreflective coating 74.

This is better shown in FIG. 8. The GRIN lens 73 collects the monitoredlight 78 from the laser diode 70. The GRIN lens 73 collimates the light.The partially reflective coating 74 applied on the end face of the GRINreflects a portion of the light back 79 while allowing another portionof the light to pass 80. The light reflected back is focused on thepower detector 77 located near the laser 77. In this configuration theGRIN lens 73 acts as both a lens and beam splitter. This wavelengthlocker can be tuned simply by moving the lens 73 in translation orrotate the assembly containing the filter 75.

Several embodiments of the present invention are specificallyillustrated and/or described herein. However, it will be appreciatedthat modifications and variations of the present invention are coveredby the above teachings and within the purview of the appended claimswithout departing from the spirit and intended scope of the invention.

1-6. (canceled)
 7. A wavelength locker comprising: a first detector; asecond detector; a collimating lens having first end to receive amonitored beam and a second end having an angled polished face to splitsaid monitored beam between said first detector and said seconddetector; and a filter between said collimating lens and at least one ofsaid first detector and said second detector.
 8. The wavelength lockerare recited in claim 7 wherein said angled polished face is a 45 degreeangle to an optical axis of said collimating lens.
 9. The wavelengthlocker as recited in claim 8 wherein said first detector is positionedalong the optical axis of said collimating lens and said second detectoris positioned perpendicular to said optical axis of said collimatinglens.
 10. The wavelength locker as recited in claim 9 wherein saidfilter is positioned between said collimating lens and said seconddetector.
 11. The wavelength locker as recited in claim 9 wherein saidfilter comprises a thin film filter applied directly to the polishedface of said collimating lens.
 12. A method of monitoring a wavelengthof a beam, comprising: providing a lens having an angled polished face;collimating a monitored beam with said lens; splitting said collimatedbeam into a first beam and a second beam with said angled polished faceof said lens; wavelength filtering at least one of said first beam andsaid second beam; detecting said first beam with a first detector tooutput a first signal detecting said second beam with a second detectorto output a second signal; and using said first signal and said secondsignal to determine a wavelength of said monitored beam.
 13. The methodof monitoring a wavelength of a beam as recited in claim 12 wherein saidangled polished face comprises a 45 degree angle to an optical axis ofsaid lens.
 14. The method of monitoring a wavelength of a beam arerecited in claim 12 further comprising: placing a wavelength filterbetween said second detector and said lens.
 15. The method of monitoringa wavelength of a beam as recited in claim 12 further comprising:placing a thin film wavelength filter directly in said angled polishedface of said lens.
 16. A wavelength locker comprising: a lens tocollimate a monitored beam from a light source; a partially reflectivecoating on one end of said lens to allow a first portion of saidmonitored beam to pass and to reflect back a second portion of saidmonitored beam through said lens; a filter to filter said first portionof said monitored beam; a first detector to detect the filtered firstportion of said monitored beam; and a second detector positionedadjacent to said light source to detect said second portion of saidmonitored beam reflected back through said lens.
 17. The wavelengthlocker as recited in claim 16 wherein said lens is a gradient index(GRIN) lens.
 18. The wavelength locker as recited in claim 16 whereinsaid partially reflective coating reflects approximately 70% of saidmonitored beam back through said lens.
 19. A method of monitoring awavelength of a monitored beam comprising: collimating a monitored beamwith a lens; allowing a first portion of said monitored beam to passthrough the lens wavelength filtering said first portion of saidmonitored beam; detecting said first portion of said monitored beam;reflecting back a second portion of said monitored beam through saidlens to produce a first signal; detecting said second portion of saidmonitored beam to produce a second signal; and using said first signaland said second signal to determine a wavelength of said monitored beam.20. The method of monitoring a wavelength of a monitored beam as recitedin claim 19 wherein said second portion of said monitored beam comprisesapproximately 70% of said monitored beam.